The present invention relates to an ink-jet recording apparatus which performs the recording by ejecting ink to a recording medium.
Conventionally, an ink-jet recording apparatus is developed in which ink is heated by a heater to generate air bubbles in the ink, and the ink is ejected with the pressure caused by the expansion of the air bubbles to a recording medium from an ejecting port of a head, thereby performing the recording.
With such an ink-jet recording apparatus, the heating of ink causes temperatures of the head and ink to change during the recording. It is observed that the temperature change of the ink causes the recorded image density to remarkably vary.
Under a high temperature environment, the ink amount of an ink drop to be formed increases in accordance with the decrease of the viscosity of ink. Therefore, dots having a larger diameter are formed on a recording medium. By contrast, under a low temperature environment, the viscosity of ink increases so that the ink amount of an ink drop to be formed decreases. Therefore, dots having a smaller diameter are formed on a recording medium. Moreover, depending on the ink temperature, the ink ejection speed also varies. As a result, the landing positions of ink drops are scattered. This largely affects the image quality. Particularly, when the print operation is halted for a long time period under the low temperature environment, the temperature of the head falls, resulting in that, when the next print operation is performed immediately after the halt, the diameter of dots constituting an image is small and the ejection performance is not stable, thereby producing a problem in that the image quality is deteriorated.
From the above, it will be noted that, if the ink temperature or head temperature is controlled so as to be kept constant throughout the printing period and the printing halt period, the ink ejection performance in the printing is stable and the ink amount of an ink drop can be made uniform, thereby eliminating the problems in the image quality such as the density variation. In the prior art, therefore, there is a problem of how to perform the recording operation while keeping the head temperature constant.
In order to solve the problem, some methods are hitherto proposed. One of the methods is disclosed in Unexamined Japanese Patent Publication No. HEI. 3-218,840. According to this method, when the recording is to be continuously performed on a plurality of recording media, the head is driven in such a degree that ink is not ejected, during a predetermined time period from the end of the recording for a first recording medium to the start of the recording for a second recording medium, thereby raising the head temperature to a prescribed temperature. Thereafter, the recording for the second recording medium is started. Therefore, the density variation among recording media can be eliminated, and a stable image quality can be always obtained even under a low temperature environment.
However, this method has a drawback that, when a low image density printing is to be performed under a low temperature environment, the head temperature is raised to the prescribed temperature at the start of the recording for one sheet, but the head temperature falls in the recording for the end portion of the sheet, thereby causing the start and end portions of the sheet to have different image densities. In such a case, furthermore, the head temperature considerably falls at the end of the printing for the sheet. This probably requires a long time period for raising the head temperature during the predetermined time period before the start of the printing of the next sheet. During this time period, it is necessary to stop the print operation, and the host computer is requested to stay in the waiting state. Therefore, this method may cause the processing speed of the whole system to reduce.
The above-identified publication discloses also that, even in the printing of one line, the ink flow path is kept heated by driving the recording head with a pulse width stored in a memory by which ink can be ejected, or by which ink cannot be ejected. However, these pulse widths are not set for every line depending on the temperature change during the recording for a single sheet. Therefore, the image density is also different between lines in a single sheet.
Further, Unexamined Japanese Patent Publication No. HEI. 1-127,361 discloses another method including a first control device for generating and supplying a drive signal for ejecting ink, and a second control device for generating and providing a drive signal by which ink cannot be ejected. Both the devices are simultaneously operated so that, in a predetermined time period, the power consumption of a nozzle which ejects ink is approximately equal to that of a nozzle which does not eject ink. Therefore, the differences in temperature between the nozzles are eliminated, and the ink amount of an ink drop is constant for both the nozzles, thereby preventing the image quality deterioration from occurring in a single sheet.
Also in this method, as in the above-mentioned prior art example, the ink ejection failure and the density variation due to the variation in the ink temperature can be prevented. However, in this method, two drive pulse generating units are provided and simultaneously operated, so that nozzles which are not ejecting ink are also driven. Therefore, the amount of currents consumed by the head is increased as a whole, producing a problem in that it requires a power source apparatus of a larger size. Moreover, in the drive method, nozzles which are not ejecting ink and nozzles which are ejecting ink are controlled so as to consume almost the same power. Therefore, all the nozzles are apparently in the state of ejecting, so that the head temperature becomes significantly high. Conversely, in this method, unless the head temperature is made high, the variation in temperature between the nozzles cannot be eliminated.
When the head temperature is too high, the ink pressure balance in the ink flow path and the bubble formation balance are lost. There are problems in that the ink ejection direction is disordered, and that external air is introduced into the ink flow path through a nozzle to make the ink ejection disabled. In view of these problems, under a high temperature environment, it is necessary to inhibit the operation of the second drive pulse generating unit, or to change the first and the second drive pulse widths so as to reduce the applied energy. In the former method, the second drive pulse generating unit becomes unnecessary. In the latter method, it is necessary to determine the first and the second drive pulse widths. When this determination is done without considering the image density, there arises a danger that the head temperature will rise still more. Therefore, the above-mentioned ink ejection is more dangerous to occur a failure. As described above, the method in which the head temperature is kept constant in a high temperature range has a problem.
Recently, it becomes possible to use a head cartridge which contains a digital circuit to allow the interface to the body of the apparatus to be simplified. Such a head is allowed to be driven through a few signal lines from the body of the apparatus. Therefore, the number of cables connecting the head to the body of the apparatus can be remarkably reduced. In such a head cartridge containing a digital circuit, nozzles which are aligned in a line are generally divided into groups each having several nozzles, and the nozzles belonging in one group are simultaneously driven so that a nozzle drive pulse width is commonly used in one nozzle group. Apparatuses in which nozzle groups are driven are disclosed in, for example, Unexamined Japanese Patent Publication No. SHO. 58-36,461. When such a head is used, it is impossible to perform the control that the divided nozzle groups are simultaneously applied with the first and second drive pulse widths which are different from each other.